Aldehydes, Ketones & Optical Isomers

A Level Chemistry

What is the difference between a aldehyde and a ketone?

Aldehydes - At least 1 Hydrogen attached to carbonyl group

Ketones - 2 Carbons attached to carbonyl group

Alcohol + acidified dichromate (VI)

Distilling a 1^o Alcohol → Aldehyde

Distilling a 2^o Alcohol → Ketone

Refluxing a 1^o Alcohol → Carboxylic acid

Are Ketones/Aldehydes soluable in water?

Yes as the C=O bond is polar, so dipole-dipole, LFs and oxygen can hydrogen bond with water

Are Ketones/Aldehydes with short or long R groups more soulable?

Short as a higher proportion of the molecule is polar

How does boiling point change as chain length of aldehydes/ketones increases?

Boiling point increases as LFs increase with chain length

Compare the boiling points of alcohols, aldehydes, ketones and alkanes

alkanes < aldehydes/ketones < alcohols

as all have LFs, only aldehydes/ketones/alcohols have dipole-dipole and only alcohols have hydrogen bonding (all between molecules)

How can you distinguish between aldehydes and ketones?

Test using a mild oxidising agent (ketones can be oxidised if vigorous enough conditions)

Describe the test with Tollen’s reagent

Silver mirror produced with an aldehyde but not a ketone

(Tollen’s reagent is ammoniacal silver nitrate)

Describe the test with Fehling’s solution

Solution contains a mild copper (II) complex ion giving a blue solution on warming, only oxidises aliphatic aldehydes, forming a orange-red precipitate of Cu₂O

Define aliphatic

Straight chain organic molecules

Describe the test with Dichromate (VI) ions

Oxidising agent which in the presence of an aldehyde turns from orange to green

What happens when aldehydes are oxidised?

They form carboxylic acids

Can ketones be oxidised?

Yes if under vigorous enough conditions, forming acids with fewer carbons

What type of reaction occurs when HCN reacts with an aldehyde/ketone?

Nucleophilic addition

What is the name of the product formed when HCN reacts with an aldehyde/ketone?

a hydroxy-nitrile such as 2-hydroxypropanenitrile

Would it be better to use HCN or KCN in a reaction with an aldehyde/ketone?

KCN as the is a higher difference in electronegativity so more CN^- ions dissociate

What can form when HCN reacts with an aldehyde/ketone?

Optical isomers as the CN^- ion can attack from above or below the plane

What can be used to reduce both aldehydes and ketones?

LiAlH₄ (Lithium aluminium hydride) (dissolved in dry ether) followed by dilute acid or water

When LiAlH₄ reacts with aldehydes/ketones what do the hydride ions act as?

Nucleophiles

What is formed when aldehydes react with LiAlH₄?

1^o alcohols (reduction)

What is formed when ketones react with LiAlH₄?

2^o alcohols (reduction)

Why would LiAlH₄ be a better reducing agent than NaBH₄?

As Aluminium is more electronegative than Boron

How can carbonyl compound be identified with 2,4-dinitrophenyl hydrazine?

2,4 dnph reacts with both aldehydes and ketones to form orange crystalline derivatives (2,4 dnp hydrazines) which have sharper and better defined melting points than the original compounds

What is the result when aldehydes react with CH₃C=O (iodoform/triiodo methane reaction)?

Only ethanal will give a positive result

What is the result when ketones react with CH₃C=O (iodoform/triiodo methane reaction)?

All ketones will give a positive

What is a positive result for the iodoform reaction?

Pale yellow precipitate formed (CHI₃)

How do you carry out the iodoform reaction?

Add the organic compound to a mixture of iodine and dilute NaOH